This invention relates to a method for converting oxydiphthalic anhydride into purified oxydiphthalic acid. The oxydiphthalic acid may be used as a curing agent for epoxy resins, and may be converted into its esters which are useful as plasticizers. In addition, the purified oxydiphthalic acid may be converted into oxydiphthalic anhydride, which is purer than the oxydiphthalic anhydride starting material. Oxydiphthalic anhydride is useful in the production of polyimide resins. Such polyimide resins have a variety of uses, but are often used in electronic application. For electronic use, it is desirable to have oxydiphthalic anhydride of high purity and low ionic concentration.
Oxydiphthalic anhydride is often prepared from 4-chlorophthalic anhydride by coupling two molecules of the 4-chlorophthalic anhydride to form oxydiphthalic anhydride. The crude product of such a coupling reaction often includes organic solvents, unreacted starting material and catalysts, ionic substances, and various colored materials of unknown composition. The process of the present invention is useful for the conversion of oxydiphthalic anhydride prepared by the coupling of two molecules of 4-chlorophthalic anhydride into pure oxydiphthalic acid. However, the process is general and may be used to convert impure oxydiphthalic anhydride from any source into pure oxydiphthalic acid. The process is particularly useful in preparing oxydiphthalic acid which is converted into oxydiphthalic anhydride for use in the production of polyimides for electronic use.
Graebe has disclosed in Leibigs Annalen der Chemie Vol. 149, p. 18, 1869, that tetrachlorophthalic acid may be purified by recrystallization from water followed by sublimation. During the sublimation process the acid is converted to the anhydride which may be reconverted to the acid by boiling in water.
U S. Pat. No. 1,301,388 discloses that phthalic anhydride may be purified by dissolving in non-aqueous solvent, and passing the solution through a material, capable of absorbing colored impurities, such as charcoal. The solution is filtered and passed through a series of crystallization tanks of successively lower temperatures. Pure phthalic anhydride is obtained in the crystallization tanks maintained at the highest temperature and the impurities are crystallized at lower temperatures.
U. S. Pat. No. 2,786,805 discloses that phthalic anhydride may be purified by mixing sufficient water with the anhydride to form a slurry, heating the slurry to 375.degree.-400.degree. F., removing the anhydride from the slurry by passing steam into the mixture and condensing the vapors at 300.degree. F. whereby a condensate of phthalic anhydride is produced which is substantially free of water. The phthalic anhydride is then further purified by fractional distillation.
U.S. Pat. No. 2,937,189 discloses that pyromellitic acid, in water solution, may be treated with activated carbon to remove any organic impurities which may be present and treated with a metal extracting agent, e.g., a cation exchange resin to remove any metals which may be present.
U.S. Pat. No. 2,985,665 discloses that phthalic anhydride may be purified by passing molten phthalic anhydride over an activated carbon bed.
U.S. Pat. No. 3,236,885 discloses that pyromellitic acid may be purified by dissolving the acid in water, treating the aqueous solution with activated carbon, separating the carbon from the aqueous solution and recovering the purified pyromellitic acid from the water.
U.S. Pat. No. 3,338,923 discloses a method of purifying pyromellitic dianhydride by treating the impure dianhydride with ketones. The patent also discloses that it is known in the prior art that pyromellitic dianhydride may be purified by converting the dianhydride into the acid with water and recrystallizing the acid from water in the presence of activated carbon.
U.S. Pat. No. 4,870,194 discloses that oxydiphthalic anhydride can be purified by filtering or centrifuging a hot solution of oxydiphthalic anhydride in a high boiling solvent to remove impurities, followed by cooling the solution to precipitate the oxydiphthalic anhydride, which can be removed from the solution by filtration or centrifuging.
U.S. Pat. No. 4,906,760 discloses that metal ion impurities may be removed from aromatic anhydrides "by refluxing the anhydride in an aqueous solution to decyclize the anhydride and to ionize or dissolve the metal impurities in the water vehicle, provide an activated adsorption agent such as activated carbon to clarify the solution, filtering off the absorption agent (and recovering the polyacid therefrom by washing the filter cake with warm water for return to the main solution) allowing the solution to stand and cool and precipitate the purified polyacid, filtering and washing the polyacid and finally recyclizing the polyacid back to the purified anhydride."
British Patent 823,507, as abstracted in CA 54:7655C, discloses that tetrachlorophthalic acid may be purified by dissolving the acid in a water solution containing 2-20% of a water miscible ether such as dioxane, tetrahydofuran and acetals. The crude tetrachlorophthalic anhydride is dissolved in a mixture of water and the ether and filtered hot. Upon cooling to room temperature, the crystals of acid form which are then washed and dried to form tetrachlorophthalic anhydride.
U.S. Pat. No. 4,914,231 discloses a method for purifying diphenylsulfone tetracarboxylic acids by dissolving the crude tetracarboxylic acid in a mixture of water and acetic acid and allowing the acid to crystallize in order to obtain a highly purified diphenylsulfone tetracarboxylic acid. The concentration of acetic acid in the solvent mixture ranges from 2 to 90 volume % and preferably from about 10 to about 70 volume %. When the crude diphenylsulfone tetracarboxylic acid contains a heavy metal ion, removal of the heavy metal ion is more effective if the solution is treated with a cation exchange resin, or with oxalic acid prior to crystallization. The Examples show that if cation exchange resins or oxalic acid are not used, the heavy metal ion level is not reduced below about 100 ppm. The amount of the solvent mixtures relative to the crude diphenyl sulfone is about 1 to 200 times and preferably 3 to 50 times.
European Patent Number 0 421 046 A1 discloses a process for producing highly pure 3,3',4,4'-biphenyl tetra-carboxylic acid or the dianhydride thereof. The 3,3',4,4'-biphenyltetracarboxylic acid is heated at a temperature of 160.degree. to 260.degree. C. in order to drive off certain impurities. In this portion of the process, the acid is cyclized to the anhydride. The anhydride is then treated with hot water at a temperature of 95.degree. to 105.degree. C. In this portion of the process, certain impurities dissolve in the water and the anhydride is converted to the acid. If the anhydride is the desired product, it may be reformed by heating the tetraacid.